Ask anyone who has had to drag a heavy object over a large distance – the bigger and heavier the object is, the harder it is to drag. This is because friction increases with size and weight for a solid, due to the contact area between object and surface being increased. When it comes to liquids, however, the picture is more complex. By filming a series of oil droplets on a surface, a group of researchers in the US has found that friction is increased if a droplet is hanging from a surface.

Incomplete picture

Rafael Tadmor, one of the researchers based at Lamar University in Texas, tells that he was driven to carry out this research by a belief that our picture of fundamental forces in liquids is still far from complete. "Rather than measuring forces directly, many scientists deduce the force from contact angles measurements," he says. Tadmor and colleagues became interested, specifically, in the interplay between gravity and liquid flow. "Why can a liquid system, a droplet for instance, be retained on a tilted surface and not yield to gravity?" he asks.

To investigate this question, the team set out to examine the behaviour of oil droplets on a flat surface. The researchers designed an experiment in which a number of oil droplets were positioned at the tip of a 1 m-long rotating arm that was made from a surface specially treated to prevent absorption. The droplets were enclosed within a sealed chamber that also contained a video camera. Tadmor hoped to capture any motion from the liquid droplets that may result from the arm rotating, a process which subjected the droplets to a centrifugal force.

In one of the configurations, Tadmor and his team compared the motion of droplets lying on top of the arm with the motion of droplets hanging from the bottom of the arm. Given that droplets would only start moving once the centrifugal force overcomes the frictional force, they could deduct a precise value for friction in each case. They discovered that the centrifugal force required to initiate sliding in the hanging droplets was 27% larger than the force required to move the droplets on the "right-side-up" droplets.

Aligning the bonds

The Lamar scientists believe that the enhanced friction for hanging droplets is due to the interaction between the oil droplet and the surface. The elongation of the hanging water droplet under the force of gravity causes the molecules at the surface of the oil droplet to be aligned in such a way that strengthens the interaction at the oil–surface interface. In the case of droplets resting on top of the arm's surface, where the droplet becomes more "squashed", gravity still enhances these forces but to a lesser extent.

"The entire way we view friction, and especially wet friction, is changed," says Tadmor, who cites the development of nano-structures in advanced computing as a field that could benefit from this research. He also believes that further fundamental research could benefit from the introduction of a new technique to measure surface interactions.

The researchers intend to now develop this research by collaborating with scientists in different research fields including researchers at the University of Georgia who are interested in the wetting properties of nano-structured surfaces. Another project with researchers at Rice University will investigate the lubrication mechanism of slime-like liquids such as that produced by snails.

This research will appear in Physical Review Letters.